1. Field of the Invention
The present invention relates to a method and apparatus for grinding wafers using a grind chuck having a high elastic modulus. The present invention relates more particularly to an apparatus and method for grinding wafers capable of protecting semiconductor chips formed on the front face of the wafer, without using an ultraviolet tape attached thereto, while protecting the semiconductor chips from being contaminated and damaged.
2. Background of the Related Art
Generally, thin semiconductor packages are produced by forming semiconductor chips on the front surface of a wafer and grinding the rear face of the wafer by means of a grinder.
A conventional wafer grinding machine includes a process chamber which provides space for performing the grinding operation. A grind table is installed partially inside and partially outside of the process chamber. A plurality of grind chucks for holding a wafer by suction force are mounted on the grind table. A first grind unit installed in the process chamber first-grinds the rear face of the wafer held by the grind chucks until the wafer has a predetermined thickness. A second grind unit grinds the first-ground rear face of the wafer again until the wafer has a desirable thickness. An intake and exhaust groove is formed through the grind table to establish a vacuum pressure to hold the wafer on the grind chuck and to exhaust air therethrough so as to release the wafer from the grind chuck. A deionized water supply duct is also employed in the wafer grinding machine to remove heat generated while the wafer is ground by spraying deionized water onto the wafer.
A porous portion filled with a plurality of pores is formed in the central region of the grind chuck. The porous portion corresponds to the intake and exhaust groove so that the wafer can be held by or detached from the grind chuck by suctioning or exhausting air. The grind chuck is formed of ceramic material having a high hardness. An upper surface of the grind chuck is periodically reground so that the flatness of the grind chuck can be consistently maintained at a fixed level.
Conventionally, an ultraviolet tape is attached to the front face of the wafer where the semiconductor chips are formed. When the rear face of the wafer is ground, the ultraviolet tape and the flow of deionized water prevent silicon dust from contacting the front face, and thereby protect the semiconductor chips formed on the front face of the wafer from being contaminated by the silicon dust. Furthermore, the ultraviolet tape has a cushioning effect which protects the semiconductor chips from stress caused by the grinding operation.
After the ultraviolet tape is attached to the front face of the wafer, the wafer is loaded on the grind chuck in such a manner that the surface of the grind chuck is in contact with the front face of the wafer. Then, a vacuum pressure is generated in the intake and exhaust groove. As a result, the wafer is held onto the grind chuck.
Thereafter, the grind chuck with the wafer held thereon is moved to the first grind unit inside the process chamber by rotating the grind table. Deionized water is then applied to the wafer. While rotating at a fixed velocity, the first grind unit moves downwardly to the wafer whereby the rear face of the wafer is ground to a predetermined thickness. Since the first grind unit rotates slowly, the first grinding operation leaves a rough rear face on the wafer.
The grind table is rotated again, and the first-ground wafer is transferred to a location beneath the second grind unit. Then, while rotating at a higher velocity than the first grind unit, the second grind unit moves downwardly to the transferred wafer in the same manner as the first grinding operation. As a result, the rear face of the wafer is secondly ground to a desirable thickness. Since the second grind unit rotates at a higher velocity than the first grind unit, the second grinding operation leave the rear face of the wafer smooth.
When the wafer is ground to the desirable thickness through the first and the second grinding operations, the wafer is unloaded from the grind chuck. The front face of the wafer is then exposed to ultraviolet rays to reduce the adhesion of the ultraviolet tape attached to the front face of the wafer. Thereafter, a removing tape having a higher adhesion than the ultraviolet tape is attached to the ultraviolet tape to remove the ultraviolet tape from the front face of the wafer.
However, such a grinding process carried out after an ultraviolet tape is attached to the front face of the wafer suffers several problems. First, since the ultraviolet tape is expensive, the manufacturing costs of the products increase. Second, the ultraviolet tape attached to the front face of the wafer incurs cumbersome additional steps of exposing the wafer to ultraviolet rays, attaching a removing tape to the ultraviolet tape, and removing the ultraviolet tape using the removing tape. This results in a complicated grinding process, increased operation time and reduced productivity,
Third, the air between the front face of the wafer and the adhesive tape is repeatedly contracted and expanded by the heat generated while the rear face of the wafer is ground and the deionized water is sprayed to remove the heat generated by the grinding. As a result, a gap is generated between the front face of the wafer and the adhesive tape. Deionized water containing silicon dust may be introduced into the gap, and the wafer may be contaminated by the silicon dust.
Fourth, since the ultraviolet tape is coated with a vinyl, static electricity occurs when the ultraviolet tape rubs against other objects. The static electricity may damage the semiconductor chips formed on the wafer.
Therefore, it is an object of the present invention to reduce manufacturing costs of semiconductor chips, simplify a wafer grinding process and prevent static electricity from occurring by removing the step of attaching an ultraviolet tape to the front face of a wafer with semiconductor chips formed thereon.
It is another object of the present invention to protect the semiconductor chips from being contaminated or damaged due to removal of the ultraviolet tape by improving the material and structure of a wafer grinding machine.
To achieve the above objects and other advantages, the present invention provides a method for grinding the rear face of a wafer. According to the method, the wafer is loaded on the grind chuck in such a manner that the front face of the wafer is in contact with the surface of the grind chuck. The wafer is held on the grind chuck by a vacuum suction force through a suction hole formed in the grind chuck. The grind chuck with the wafer held thereon moves toward a first grind unit. At the same time that deionized water is sprayed at the rear face of the wafer to remove heat generated while the wafer is being ground, deionized water or air rises in the direction opposite to the deionized water supplied direction through a rising groove formed along the edge of the grind chuck. After the wafer is ground to the first thickness by a first grind unit, the wafer is transferred to a location corresponding to a second grind unit. Then, the wafer is ground again to a second thickness, i.e., a desirable thickness, by the second grind unit in the same manner as the first grinding step. When the wafer is completely ground to the desirable thickness, the wafer is unloaded from the grind chuck.
The present invention also provides a wafer grinding machine for performing the above steps. The wafer grinding machine comprises: a process chamber for providing a space for performing a wafer grinding process; a grind table installed partially inside and partially outside of the process chamber; a plurality of grind chucks mounted on the grind table for holding the wafer thereon by a vacuum suction force; a deionized water supply duct for applying deionized water to the wafer held by the grind chuck; a first grind unit for grinding the wafer to a first thickness; and a second grind unit for grinding the first-ground wafer to a second thickness. The grind chuck includes a body having a predetermined thickness and a shape corresponding to the wafer; a ring-shaped porous portion formed in the central region of the body and including a plurality of suction pores; and a ring-shaped rising groove formed along the peripheral edge of the body. A grind chuck accommodating groove for accommodating the grind chuck is formed in the grind table. Corresponding to the porous portion, an intake and exhaust groove is formed in the grind chuck accommodating groove. The rising groove is connected to a fluid supply duct formed in the grind chuck accommodating groove through a rise hole formed through the body of the grind chuck. The intake and exhaust groove is connected to a vacuum pump and a blower through an intake and exhaust hole formed through the grind table.
Preferably, a connecting duct projects from the bottom of the grind chuck accommodating groove and connects the fluid supply duct to the rise hole. A plurality of coupling projections are formed on the bottom of the body. A plurality of coupling grooves corresponding to the plurality of coupling projections are formed in the bottom of the grind chuck accommodating groove.
Preferably, the inner diameter of the ring-shaped rising groove is equal to or a little smaller than the diameter of the wafer.
In addition, preferably, deionized water having a resistivity of 16 M or air is supplied through the fluid supply duct.
Preferably, the grind chuck is formed of polytetrafluoruethylene or rubber that is a soft material having a high elastic modulus.
Another embodiment of the wafer grinding machine includes a process chamber for providing a space for performing a wafer grinding process; a grind table installed partially inside and partially outside of the process chamber; a plurality of grind chucks mounted on the grind table for holding the wafer thereon by a vacuum suction force; a deionized water supply duct for applying deionized water to the wafer held by the grind chuck; a first grind unit for grinding the wafer to a first thickness; and a second grind unit for grinding the first-ground wafer to a second thickness; a dam surrounding the grind chuck forming a space in which the first and second grind units can operate; and a deionized water exhaust hole formed through the grind table in the space defined by the dam. The grind chuck includes a body having a predetermined thickness and a shape corresponding to the wafer; and a ring-shaped porous portion formed in the central region of the body and having a plurality of suction pores. A grind chuck accommodating groove for accommodating the grind chuck is formed in the grind table. Corresponding to the porous portion, an intake and exhaust groove is formed in the grind chuck accommodating groove. The intake and exhaust groove is connected to a vacuum pump and a blower through an intake and exhaust hole formed through the grind table. Preferably, first and second branch ducts for applying deionized water to the wafers corresponding to the first and the second grind units, respectively, are integrally connected to an end of the deionized water supply duct.
Preferably, the space formed by the dam fills with deionized water from the deionized water supply duct such that the wafer held by the grind chuck is submerged in the deionized water within the dam.